406 On the Causes of Single and Erect Vision, 



duced by curiosity only to look it over ; but to any one who 

 means to put it in practice, the whole will be found essential, 

 and with a little attention and experience become familiar and 

 easy, and in which I have endeavoured to combine every ad- 

 vantage the subject will admit of; and as coming from the 

 w fountain head," it may not prove uninteresting to some at least 

 of your numerous readers. I am, Gentlemen, 



Your most obedient servant, 

 Oxford, April 28, 1828. RlCHARD Walker. 



LXIII. On the Causes of Single and Erect Vision*. By 



FN order to understand aright the reason of single and erect 

 *~ vision, it is necessary first of all to perceive the truth of 

 certain metaphysical positions in relation to vision, without 

 the establishment of which, confused ideas, hypothetical 

 assumptions, and inconclusive reasonings on optical experi- 

 ments and facts are presented to the mind, and tend to em- 

 barrass the simplicity of that truth which might otherwise be 

 immediately revealed. 



First, — Vision is a consciousness in the mind, and its next 

 proximate cause must be a power equal to its production, and 

 which unites it to the material world. 



Secondly,— Vision of one colour only can never yield the 

 vision of figure, because the proximate cause of the vision of 

 figure is a line of demarcation formed by the sensation of the 

 junction of two colours. 



Thirdly, — The physical impulse producing such conscious- 

 ness of colouring, is an equal proportional variety upon the 

 retina of an eye ; one eye alone being first supposed, as it is 

 truly efficient to yield the idea of figure. 



Fourthly, — An object cannot be in two places at the same time. 



Fifthly, — An object cannot exist and put forth its action 

 wheie it is not. 



These premises being supposed to be granted, let the ques- 

 tion be asked, Why with two eyes given, two objects are 

 not seen, although there be but one object given externally ? 



The answer (when supported by the foregoing premises, 

 and conjoined with certain optical facts with which all 

 who are conversant with the subject, are acquainted) will 

 be, became there is not presented to the mind that variety 

 of colouring which is necessary and alone efficient as the next 

 proximate cause of vision; that is, there are not two lines of 

 separate demarcation between two objects, but one line qfdemar- 



* Communicated by a friend of the Author. 



f Author of " An Essay on the Relation of Cause and Effect " and of 

 " Essays on the Perception of an External Universe," &c. 



cation 



